A conventional inkjet printing system includes a printhead, an ink supply that supplies liquid ink to the printhead, and an electronic controller that controls the printhead. The printhead ejects ink drops through a plurality of orifices or nozzles toward a print medium, such as a sheet of paper, so as to print onto the print medium. Typically, the orifices are arranged in one or more arrays such that properly sequenced ejection of ink from the orifices causes characters or other images to be printed upon the print medium as the printhead and the print medium are moved relative to each other.
One type of inkjet printing system is an inline printing system in which one or more printheads are fixed and the print medium is moved relative to the printhead(s). The speed of the print medium relative to the printhead(s) is measured by an encoder. In addition, the encoder tracks the position of the print medium relative to the fixed printheads with a resolution typically indicated in dots per inch (dpi). Typically, for multiple printhead inline printing systems, the image to be printed is divided between two or more printheads by a multiple of the encoder resolution. By dividing the image to be printed into multiple images, the firing frequency of each printhead is reduced. Therefore, the print medium can be moved faster relative to the printheads while having the same final number of drops ejected onto the print medium. For example, in an inline printing system having four printheads and a 150 dots per inch (dpi) encoder, a 600 dpi image to be printed can be divided into four 150 dpi images or two 300 dpi images that are printed interlaced to provide the final desired 600 dpi image.
The printheads have a firing frequency that ranges from zero to a maximum value, such as 36 kHz. In one embodiment, the firing frequency in kHz is defined by the following Equation I:
                    kHz        =                              fpm            5                    ×                      hRes            1000                                              Equation        ⁢                                  ⁢        I            
where:                fpm=feet per minute of the print medium relative to the printheads; and        hRes=the horizontal resolution of the image.        
Typically, in inline printing systems, an encoder is used to measure the speed of the print medium relative to the printhead(s) to set the firing frequency of the printhead(s) needed to obtain the desired resolution. For example, in a 600 dpi printing system having a 150 dpi encoder, one printhead can be used to print at 600 dpi to obtain a final resolution of 600 dpi. The maximum speed of the print medium to print a 600 dpi image using one printhead at a firing frequency of 36 kHz is 300 fpm. The maximum speed of the print medium to print a 600 dpi image using two printheads printing interlaced 300 dpi images at a firing frequency of 36 kHz is 600 fpm. The maximum speed of the print medium to print a 600 dpi image using four printheads printing interlaced 150 dpi images at a firing frequency of 36 kHz is 1200 fpm.
Typically, printheads have a range of values in the middle of the firing frequency range, such as 12 kHz to 24 kHz, where the printheads do not provide a good quality image. The firing frequency interval where the printheads do not provide a good quality image is called the “puddling zone” and should be avoided to obtain good image quality and printhead reliability. Therefore, to avoid the puddling zone, typically certain speeds of the print medium that would require the printheads to fire at a firing frequency within the puddling zone are avoided. In some circumstances, however, it is undesirable to avoid printing at certain speeds, such as where another system controls the print medium speed.
For these and other reasons, there is a need for the present invention.